The present invention relates to a method of manufacturing a golf ball having a cover formed from a thermoplastic resin, and particularly to a method of manufacturing a golf ball in which the surface layer of a cover formed from a thermoplastic resin is modified to impart excellent characteristics to the cover.
Solid golf balls such as two-piece golf balls and thread-wound golf balls are usually manufactured by the steps of enclosing a solid core or thread-wound core with a cover by compression molding or injection molding, forming dimples in the cover, and stamping markings on and applying a coating onto the surface of the cover. In this manufacturing process, thermoplastic resins such as ionomer resins, polyolefin resins, and polyurethane resins are often used as materials for covers.
When thermoplastic resins are used as materials for covers of golf balls, covers and dimples in covers can be readily formed by compression molding or injection molding. However, covers formed from thermoplastic resins may be unsatisfactory in heat resistance, wear resistance, compression resistance, solvent resistance, chemical resistance, or like physical properties. Also, imparting a certain performance to a cover is impossible unless a thermoplastic resin used can impart the property to the cover.
By contrast, use of thermosetting resins as materials for covers of golf balls enables an improvement of physical properties of covers and can impart to covers performance which thermoplastic resins cannot. However, use of thermosetting resins as cover materials limits the method of molding to compression molding and involves difficulty in setting molding conditions. As a result, the formation of covers and dimples becomes difficult to carry out.
In recent years, in order to meet the demand for improved resilience, spin properties, and feel on impact, covers have employed a multilayered structure consisting of a plurality of layers having different physical properties, i.e. multi-piece covers have been employed. However, even in the case of using either thermoplastic resins or thermosetting resins as materials for covers, constituent layers of a multilayered cover are difficult to be uniformly formed to a thickness of not greater than 1 mm. Consequently, the thickness of each layer or the overall thickness of a cover becomes relatively thick, resulting in a failure to obtain desired performance.
In view of the foregoing, an object of the present invention is to provide a method of manufacturing golf balls capable of improving physical properties of the cover of a golf ball, imparting to the cover performance which cannot be imparted by thermoplastic resins alone, and reducing the thickness of each layer of a multilayered cover or the overall thickness of the cover, by modifying the surface layer of a cover formed from an easy-to-mold thermoplastic resin.
To achieve the above object, the present invention provides a method of manufacturing golf balls, comprising the steps of: molding a cover through use of either a thermoplastic resin having a group capable of reacting with an isocyanate group or a thermoplastic resin containing a compound having two or more groups capable of reacting with an isocyanate group; and causing a polyisocyanate compound to permeate into the surface layer of the cover to thereby induce the reaction between the polyisocyanate compound and the group(s) capable of reacting with the isocyanate group, whereby a modified layer is formed at the surface of the cover.
According to the method of the present invention, a cover is molded from either a thermoplastic resin having a group (a reactive group) capable of reacting with an isocyanate group or a thermoplastic resin containing a compound (a reactive compound) having two or more reactive groups. Subsequently, a polyisocyanate compound is caused to permeate into the surface layer of the cover to thereby cause the reaction between the polyisocyanate compound and the reactive group(s) contained in the thermoplastic resin, causing a number of urethane linkages and urea linkages, i.e. a bridge structure. Thus is a modified surface layer. Through appropriate selection of the above thermoplastic resin, reactive groups, reactive compound, and polyisocyanate compound, desired physical properties can be imparted to the surface layer of a cover.
In the present invention, a golf ball is manufactured according to, for example, the procedure shown in FIGS. 1A and 1B. First, as shown in FIG. 1A, a solid core 2 formed from polybutadiene rubber or the like is enclosed with a single-layer cover 4 through injection molding or a like molding of a thermoplastic resin. Next, as shown in FIG. 1B, a polyisocyanate compound is caused to permeate into the surface layer of the cover 4 to thereby cause the reaction between the penetrating polyisocyanate compound and a reactive group contained in the thermoplastic resin of the cover 4, whereby the surface layer is modified to obtain a modified layer 6.